Technical Field
This disclosure relates to object scanners, in particular, but not restricted to a specular object scanner for measuring reflectance properties of objects.
Description of Related Art
An extensive body of work in the graphics and vision literature addresses the acquisition of geometry and reflectance from images under controlled and uncontrolled lighting conditions. Two recent overviews are Weyrich, T., Lawrence, J., Lensch, H. P. A., Rusinkiewicz, S., and Zickler, T. 2009, Principles of appearance acquisition and representation, Found. Trends. Comput. Graph. Vis. 4, 2 (Feb.), 75-191, which covers a wide variety of techniques for acquiring and representing BRDFs and Spatially Varying BRDFs (SVBRDFs) over object surfaces, and Ihrke, I., Kutulakos, K. N., Lensch, H. P. A., Magnor, M., and Heidrich, W. 2010, Transparent and specular object reconstruction, Computer Graphics Forum 29, 8, 2400-2426, which focused on the acquisition of on purely specular and transparent objects.
SVBRDFs can be captured exhaustively using point light sources (e.g. Dana, K. J., Van Ginneken, B., Nayar, S. K., and Koen Derink, J. J. 1999, Reflectance and texture of real-world surfaces. ACM Trans. Graph. 18, 1 (Jan.), 1-34; McAllister, D. K. 2002, A generalized surface appearance representation for computer graphics, PhD thesis, The University of North Carolina at Chapel Hill. AAI3061704, but this requires a large number of high-dynamic range photographs to capture every possible combination of incident and radiant angle of light. Similar to the disclosed approach, many techniques (e.g. Debevec, P., Hawkins, T., Tchou, C., Duiker, H.-P., Sarokin, W., and Sagar, M. 2000, Acquiring the reflectance field of a human face, in Proceedings of ACM SIG GRAPH 2000, 145-156; Gardner, A., Tchou, C., Hawkins, T., and Debevec, P. 2003, Linear light source reflectometry, in ACM TOG, 749-758.); Holroyd, M., Lawrence, J., Humphreys, G., and Zick LER, T. 2008, A photometric approach for estimating normal and tangents, ACM Trans. Graph. 27, 5 (Dec.), 133:1-133:9; Ren, P., Wang, J., Snyder, J., Tong, X., and Guo, B. 2011. Pocket reflectometry, ACM Trans. Graph. 30, 4 (Jul.), 45:1-45:10 look instead at BRDF slices of spatially-varying materials observed from a single viewpoint to infer parameters of a reflectance model, which can be used to extrapolate reflectance to novel viewpoints. Other approaches Sato, Y., Wheeler, M. D., and Ikeuchi, K. 1997, Object shape and reflectance modeling from observation, in Proceedings of the 24th annual conference on Computer graphics and interactive techniques, ACM Press/Addison-Wesley Publishing Co., New York, N.Y., USA, SIGGRAPH '97, 379-387; Lensch, H. P. A., Kautz, J., Goesele, M., Heidrich, W., and Seidel, H.-P. 2003, Image-based reconstruction of spatial appearance and geometric detail, ACM TOG 22, 2, 234-257; Zickler, T., Ramamoorthi, R., Enrique, S., and Bel Humeur, P. N. 2006, Reflectance sharing: Predicting appearance from a sparse set of images of a known shape., PAMI 28, 8, 1287-1302; Holroyd, M., Lawrence, J., and Zickler, T. 2010, A coaxial optical scanner for synchronous acquisition of 3d geometry and surface reflectance, ACM Trans. Graph. 29, 4 (Jul.), 99:1-99:12 use sparse sets of viewpoint and lighting directions and extrapolate BRDFs per surface point assuming that the reflectance varies smoothly over the object. This approach is also used by Dong, Y., Wang, J., Tong, X., Snyder, J., Lan, Y., Ben Ezra, M., and Guo, B. 2010, Manifold bootstrapping for SVBRDF capture, ACM Trans. Graph. 29 (Jul.), 98:1-98:10, which employs a dedicated BRDF measurement system to sample representative surface BRDFs, which are extrapolated to the surface of the entire object based on its appearance under a moderate number of environmental lighting conditions. None of these techniques, however, produces independent measurements of diffuse and specular reflectance parameters for each observed surface point, and thus may miss important surface reflectance detail.
Ikeuchi, K. 1981, Determining surface orientations of specular surfaces by using the photometric stereo method, IEEE Trans. Pattern Anal. Mach. Intell. 3, 6 (Jun.), 661-669 extended the original photometric stero approach of Woodham, R. J. 1980, Photometric method for determining surface orientation from multiple images, Optical Engineering 19, 1, 139-144 to specular surfaces, using a set of angularly varying area light sources to estimate the specular surface orientation. Nayar, S., Ikeuchi, K., and Kanade, T. 1990, Determining shape and reflectance of hybrid surfaces by photometric sampling, IEEE Trans. Robotics and Automation 6, 4, 418-431 used an extended light source technique to measure orientations of hybrid surfaces with both diffuse and specular reflectance, but they did not characterize the BRDF of the specular component. Gardner, A., Tchou, C., Hawkins, T., and Debevec, P. 2003, Linear light source reflectometry, in ACM TOG, 749-758.; Holroyd, M., Lawrence, J., Humphreys, G., and Zick L E R, T. 2008, A photometric approach for estimating normal and tangents, ACM Trans. Graph. 27, 5 (Dec.), 133:1-133:9 employed a moving linear light source to derive BRDF models of spatially-varying materials, including highly specular materials, but still required hundreds of images of the moving light to record sharp reflections. Hawkins, T., Einarsson, P., and Debevec, P. 2005, A dual light stage, in Proc. EGSR, 91-98 recorded diffuse and specular reflectance behavior of objects with high angular resolution using a surrounding spherical dome and a laser to excite the various surface BRDFs through Helmholz reciprocity, but achieved limited spatial resolution and required a high-powered laser equipment. Recently, Wang, C.-P., Snavely, N., and Marschner, S. 2011, Estimating dual-scale properties of glossy surfaces from step-edge lighting, ACM Trans. Graph. (Proc. SIGGRAPH Asia) 30, 6 used step-edge illumination to estimate dual scale reflectance properties of highly glossy surfaces, but did not estimate per-pixel BRDFs.
Ma, W.-C., Hawkins, T., Peers, P., Chabert, C.-F., Weiss, M., and Debevec, P. 2007, Rapid acquisition of specular and diffuse normal maps from polarized spherical gradient illumination, In Rendering Techniques, 183-194 used spherical gradient illumination representing the 0th and 1st order Spherical Harmonics in an LED sphere to perform view-independent photometric stereo for diffuse and/or specular objects, and used polarization difference imaging to independently model diffuse and specular reflections of faces. Ghosh, A., Chen, T., Peers, P., Wilson, C. A., and Debevec, P. E. 2009, Estimating specular roughness and anisotropy from second order spherical gradient illumination, Comput. Graph. Forum 28, 4, 1161-1170 extended this approach by adding 2nd order Spherical Harmonics to estimate spatially varying specular roughness and anisotropy at each pixel. Unfortunately, the use of an LED sphere with limited resolution made the reflectance analysis applicable only to relatively rough specular materials such as human skin, and the use of polarization for component separation becomes complicated for metallic surfaces and near the Brewster angle. To avoid using polarization for reflectance component separation, Lamond, B., Peers, P., Ghosh, A., and Debevec, P. 2009, Image-based separation of diffuse and specular reflections using environmental structured illumination, in Proc. IEEE International Conf. Computational Photography modulated gradient illumination patterns with phase-shifted high-frequency patterns to separate diffuse and specular reflections and measure surface normals of 3D objects. The subject technology reformulates and generalizes this frequency-based component separation approach to increasing orders of spherical harmonic illumination. Noting that BRDFs can usefully represented by spherical harmonic functions (e.g. Westin, S. H., Arvo, J. R., and Torrance, K. E. 1992, Predicting reflectance functions from complex surfaces, SIGGRAPH Comput. Graph. 26, 2 (Jul.), 255-264), Ghosh, A., Heidrich, W., Achutha, S., and O'Toole, M. 2010, A basis illumination approach to brdf measurement, Int. J. Comput. Vision 90, 2 (Nov.), 183-197 used spherical harmonic illumination projected to a zone of a hemisphere for reflectance measurement, but only for single BRDFs from flat samples. Their approach also did not separate diffuse and specular reflectance using the measurements and required very high orders of zonal basis function to record sharp specular reflectance. Instead, in the disclosed work employing up to 5th order spherical harmonics is proposed for both diffuse-specular separation as well as estimating reflectance statistics such as specular roughness and anisotropy.
Highly specular objects have long posed problems for image-based shape reconstruction Blake, A., and Brelstaff, G. 1992, in Physics-Based Vision, Principles and Practice: Shape Recovery, L. B. Wolff, S. A. Shafer, and G. E. Healey, Eds. Jones and Bartlett Publishers, Inc., USA, ch. Geometry from specularities, 277-286. Bonfort, T., and Sturm, P. 2003, Voxel carving for specular surfaces, In Proc. IEEE International Conference on Computer Vision, 591-596 pro-posed a multi-view voxel carving technique based on reflected observations of a calibrated world pattern, but achieved very low resolution results. Tarini, M., Lensch, H. P., Goesele, M., and Seidel, H.-P. 2005, 3D acquisition of mirroring objects using striped patterns, Graphical Models 67, 4, 233-259 proposed an alternate shape from distortion approach for high resolution reconstruction. Using a CRT screen as an extended light source, they illuminate a specular object with several stripe patterns to obtain first a matte and then iteratively solve for the depth-normal ambiguity. Chen, T., Goesele, M., and Seidel, H. P. 2006, Mesostructure from specularities, in CVPR, 1825-1832 propose measuring mesostructure from specularity using a hand held light source that is waved around while a camera observes the moving specular highlights on the sample to estimate its surface normals. Francken, Y., Cuypers, T., Mertens, T., Gielis, J., and Bekaert, P. 2008, High quality mesostructure acquisition using specularities, CVPR, 1-7 propose measuring surface mesostructure instead by using a set of calibrated gray codes projected from an LCD panel in order to localize the surface normal of each surface point. These techniques work well in principle but are limited to scanning small flat objects that are covered by the illumination from an extended source. Adato, Y., Vasilyev, Y., Ben-Shahar, O., and Zickler, T. 2007, Toward a theory of shape from specular flow, In Proc. IEEE International Conference on Computer Vision, 1-8 have proposed an alternate approach for shape reconstruction by formulating a set of coupled PDEs based on observed specular flow at a specific surface point. They also derive a simple analytic formulation for a special case of camera rotation about the view axis. While having the advantage of not requiring control over the incident illumination, in practice the method yields only very simple shape reconstructions. In contrast, the disclosed technique combines cues from both diffuse and specular reflectance information to derive high-fidelity geometric models for many common types of objects.